1. Field of the Invention
This invention relates to a method and apparatus for low frequency eddy current testing using a magnetometer, and preferably a SQUID magnetometer, to detect flaws on or below the surface of an electrically conductive object.
2. Background Information
Conventional eddy current techniques, which are very sensitive for detecting surface breaking cracks in metal, have been studied and developed extensively for many years. Because of the frequency-dependent skin depth of eddy currents, the detection of deep, non-surface-breaking flaws requires very low frequency eddy currents. However, the key parameters measured by eddy current probes, typically the secondary voltage induced in the pick-up coil or the impedance change of the probe, resulting from eddy currents induced in the conductor by the primary coil, are proportional to the frequency of the magnetic field to be measured. Hence the sensitivity and the signal-to-noise ratio are suppressed drastically at the low frequencies that are required for detection of subsurface cracks. To obtain high sensitivity, most eddy current probes operate at frequencies of tens to hundreds of kilohertz, and the corresponding skin depth in aircraft-grade aluminum alloys varies from 1.2 mm (0.05 in) to 0.2 mm (0.01 in). This is a particularly significant problem in the detection of second and third-layer cracks and corrosion in aging aircraft, and in determining the existence or size of voids in thick soldered assemblies such as splices for electrical generator windings.
As an alternative electromagnetic approach, ac field techniques have been developed during the past decade. Instead of inductively coupling the electromagnetic field above the metal surface as in eddy current methods, the differences in surface electric potential due to a current injected or induced inside the metal are measured by using two electrodes in direct contact with the metal surface. This technique is suitable for a conducting object with a smooth conducting surface, and may be used for dc or low frequency testing. However, the current disturbed by the flaw is constrained largely within a region of comparable dimensions to those of the flaw. A flaw hidden beneath the surface may not disturb the surface potential significantly, even at a low enough frequency, if the distance between the flaw and the surface is larger than the dimensions of the flaw. Thus, the detectability of subsurface flaws is limited.
A modified inducing mechanism for eddy current technique, which consists of two parallel U-shaped wires, has been devised to simplify the complicated calibration procedure in the ordinary eddy current method, but this inductive approach is as yet limited to surface-breaking cracks in ferromagnetic material.
High resolution SQUID (Superconducting Quantum Interference Device) magnetometers, which are very sensitive to dc and low frequency magnetic fields, have been developed and used for detection of flaws in non-ferromagnetic conductors. By injecting a spatially uniform current (dc or low frequency ac current) into a conductor, and measuring the magnetic field normal to the surface, subsurface flaws can be detected. The magnetic field near the surface due to a subsurface flaw is dependent not only on the surface current distribution, as is the case in ac field measurements, but also on the total current disturbed by the flaw inside the conductor.
It is difficult to inject current in several circumstances, such as a conductor covered by an insulation layer, or a cylindrical tube with a crack along the tube axis which requires currents in the azimuthal direction. In these cases, induced eddy currents may be used instead of injected current for SQUID NDE (non-destructive evaluation). In contrast to the eddy current probe, the amplitude of the output signal of the SQUID is independent of the frequency of the field measured, so extremely low frequency (ELF) eddy current measurements are feasible. However, the small excitation coils used in most eddy current techniques induce localized eddy currents circulating inside the conductors, which produce a large magnetic field in the direction normal to the surface. This kind of excitation coil is disadvantageous for a SQUID magnetometer, whose pick-up coil is typically sensitive to the field component normal to the surface, since the magnetic signals due to the current disturbed by flaws in a conductor are difficult to discriminate from the large field background resulting from the large eddy currents circulating beneath the excitation coil.
There is a need therefore, for an improved method and apparatus for detecting sub-surface flaws in electrically conductive objects, including tubular electrically conductive objects.
There is a related need for such a method and apparatus which can detect sub-surface flaws despite the presence of surface flaws.
There is a need for such a method and apparatus which is simple and economical to implement.